As an exhaust emission control apparatus provided in an exhaust passage of an internal combustion engine, an exhaust emission control apparatus in which a selective reduction type NOx catalyst (hereinafter referred to as SCR catalyst) is carried on a filter has been developed (see, for example, Patent Document 1). The filter collects particulate matter (hereinafter referred to as PM) in exhaust gas. The SCR catalyst reduces NOx in the exhaust gas using ammonia (NH3) as a reducing agent. Such a filter carrying the SCR catalyst is hereinafter sometimes referred to as SCRF.
By adopting the SCRF as the exhaust emission control Apparatus, it is possible to reduce the size of the exhaust emission control apparatus compared with when the filter and the SCR catalysis are separately provided in the exhaust passage. Therefore, it is possible to improve mountability of the exhaust emission control apparatus. Further, by adopting the SCRF, it is possible to dispose the SCR catalyst further on an upstream side in the exhaust passage. As the SCR catalyst is disposed further on the upstream side in the exhaust passage, the SCR catalyst is more easily heated by the heat of the exhaust gas. Therefore, it is possible to attain improvement of warming-up properties of the SCR catalyst and improvement of an NOx removal rate in the SCR catalyst.
The collected PM deposits on the SCRF. Therefore, filter regeneration treatment is executed in the exhaust emission control system including the SCRF. The filter regeneration treatment is treatment for oxidizing the PM deposited on the SCRF and removing the PM. The filter regeneration treatment is realized by supplying fuel (HC) to a first stage catalyst, which is a catalyst provided in the exhaust passage upstream of the SCRF and having an oxidation function. When the fuel is oxidized in the first stage catalyst, the exhaust gas flowing into the SCRF is heated by oxidation heat. Therefore, it is possible to increase the temperature of the SCRF to a filter regeneration temperature at which the oxidation of the PM is facilitated.
Note that it is known that an HC adding valve for adding the HC and a urea adding valve for adding urea water are provided in the exhaust passage upstream of the SCR catalyst and, when a urea water amount necessary for removing the NOx by the SCR catalyst exceeds a predetermined upper limit amount, the HC is supplied in addition to the supply of the urea water (see, for example, Patent Document 2).
Incidentally, in the exhaust emission control system including the SCRF, when the filter regeneration treatment is carried out, a large amount of the HC is supplied to the first stage catalyst. In the conventional exhaust emission control system in which the SCR catalyst is provided downstream of the filter, most of the HC is oxidized in the first stage catalyst and the filter during the filter regeneration treatment. Therefore, an amount of the HC flowing into the SCR catalyst is relatively small. However, since only the first stage catalyst is present upstream of the SCRF, it is likely that the HC not fully oxidized by the first stage catalyst flows into the SCRF. When the HC flows into the SCRF, a part of the HC reduces the NOx. Therefore, when ammonia is supplied on the basis of an amount of the NOx flowing into the SCRF, the ammonia is oversupplied by an amount of the NOx reduced by the HC. That is, the ammonia more than necessary for reducing the NOx is supplied. Since the excess ammonia flows out from the SCRF, it is likely that the ammonia is wasted.